This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for MANUFACTURE METHOD FOR SILICA GLASS OF TUBE TYPE earlier filed in the Korean Industrial Property Office on the Aug. 26, 1999 and there duly assigned Serial No. 35602/1999.
1. Field of the Invention
The present invention relates to silica glass and to a method for manufacturing silica glass, and more particularly to silica glass products and to a method for manufacturing a tube-shaped silica glass product.
2. Description of the Prior Art
In the manufacture of optical fiber, which is a high-speed and low-loss data transmission medium, both methods of directly drawing an optical fiber from a liquid material, for example, a double crucible process, and methods of drawing the optical fiber from a rod-shaped preform are known. Among methods of drawing an optical fiber from a preform, various methods respectively using different preform formation processes are known; for instance, the modified chemical vapor-phase deposition (MCVD) process. In accordance with this method, a preform is formed by depositing a vapor-phase material on the inner or outer surface of a substrate tube.
Another method involves forming a preform by molding a core rod and an over-jacketing tube, and interconnecting the rod and tube to form the preform. Both the substrate tube of the modified chemical vapor-phase deposition process and the over-jacket tube are made of silica glass in the form of tubes, and these are usually made by using a sol-gel method.
In a general method of manufacturing a preform, a silica glass tube is manufactured using a mixing/dispersing process, a molding process, a de-molding process, a drying process, an organic substance treating process, and a sintering process. An example of this method is disclosed in U.S. Pat. No. 5,240,488, to Chandross et al., entitled Manufacture Of Vitreous Silica Product Via A Sol-Gel Process Using A Polymer Additive.
In the mixing/dispersing procedure, a starting material is mixed with deionized water and an additive such as a dispersing agent, so that the starting material is uniformly dispersed in the deionized water, thereby forming a uniform sol. The starting material may include silicon alkoxide or fumed silica. A dispersion process using silicon alkoxide is disclosed in U.K. Patent No. 2,041,913. A dispersion process using fumed silica is disclosed in U.S. Pat. No. 4,419,115, to Johnson, Jr. et al, entitled Fabrication Of Sintered High Silica Glasses.
In the molding procedure, the sol produced in the mixing/dispersing procedure is charged into a mold having a proper shape for gelation. The sol is added with a binder and a gelling agent to aid in forming particle-to-particle bonds. The mold, which is used to mold a silica glass tube, for example, a substrate tube or an over-jacket tube, has a construction including a cylindrical portion and a central rod portion inserted in the cylindrical portion.
In the de-molding procedure, the gel molded to have a tube shape in the molding procedure is removed from the mold. This procedure may be carried out in a water tank to prevent the gel tube from being damaged.
In the drying procedure, the gel tube removed from the mold is dried using a dryer with a constant temperature and humidity chamber.
In the organic substance treating procedure, the gel tube is subjected to a thermal treatment at a low temperature in order to remove or decompose residual moisture and any organic substances, such as a binder, present in the gel tube. And then, the gel tube is heated in an atmosphere of chlorine (Cl) gas to remove metallic impurities and hydrides. As a result, a molded green body is formed. This green body is a gel body molded to have a tube shape and is not yet subjected to a sintering process.
In the sintering procedure, the molded green body produced by the drying procedure is sintered so that it is glassified. As a result, a desired silica glass product is obtained. This procedure is carried out by heating the dried and impurity-removed gel to a temperature of 1,350xc2x0 C. to 1,400xc2x0 C. in a sintering furnace in an atmosphere of He gas.
The sintering procedure may be carried out using a sintering furnace. For the sintering procedure, the molded green body is vertically arranged in such a fashion that it is partially disposed within the sintering furnace. As the sintering furnace moves downward at a constant speed under the condition in which the temperature of the sintering furnace is increased to a reference temperature, the molded green body is locally glassified by virtue of heat generated in the sintering furnace. That is, the sintering procedure for the molded green body is carried out in a local fashion by the sintering furnace. In this regard, this sintering procedure is called a zone-sintering process. After completion of the sintering procedure, a tube-shaped silica glass product of a high purity such as a substrate tube or an over-jacket tube is obtained.
The molded green body that is formed in accordance with the above mentioned conventional method has a hollow rod, or tube, structure in which the inner diameter d3 at the top surface is the same as the inner diameter d1 at the bottom surface, and the outer diameter d4 at the top surface is the same as the outer diameter d2 at the bottom surface. However, the silica glass tube, which is produced from the molded green body having the above mentioned shape, in accordance with the sintering process involved in the above mentioned conventional method, has a tapered structure deformed from that of the molded green body in such a fashion that the outer diameter D1 at the bottom surface is more than the outer diameter D2 at the top surface. This is because the molded green body, vertically arranged in the sintering furnace, elongates downward due to the weight of the body, so that the gel particles of the molded green body are rendered to flow downward.
For example, in the case of an over-jacketing tube formed using a molded green body of 7 kg for the manufacture of an optical fiber of 250 km, the longitudinal cross section distribution may exceed a tolerance limit. The longitudinal cross section distribution value indicates the amount of deformation of the inner and outer diameters over a given axial range. The percentage value of longitudinal cross-section is calculated by {(Maximum Cross-sectional Areaxe2x88x92Minimum Cross-sectional Area)/Average Cross-sectional Area}xc3x97100. Here, maximum cross-sectional area occurs at the bottom, where the diameter is greatest, and minimum cross-sectional area occurs at the top, where the diameter is smallest. The tolerance limit value, that is, the maximum value of longitudinal cross-section distribution allowing an acceptable silica glass tube product, is determined by trial and error. A tolerance limit of 5% is a typical limit.
Additional examples of optical fiber preform manufacture of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 5,352,259, to Oku et al., entitled Method Of Manufacturing Optical Fiber Preform, describes a method in which a rod member made of a silica-based material is disposed within a mold cavity, a molding material is loaded within the cavity and pressure is applied.
U.S. Pat. No. 5,769,921, to Yokokawa, entitled Method Of Producing Quartz Glass Body, describes a method for producing a rod or tube for an optical fiber preform. The method involves rotating a solid or hollow cylindrical substrate which has a tapered shape, and forming a porous quartz glass body on the outer surface of the substrate.
U.S. Pat. No. 6,091,500, to Bahr et al., entitled Method And Apparatus For Measuring Overclad Tubes, describes an apparatus for measuring parameters of an overclad tube, including the outside diameter and inside diameter along the length of the tube.
Where an optical fiber is manufactured using a silica glass tube with a longitudinal cross section distribution exceeding a tolerance limit, it may have a non-uniform geometric structure in a longitudinal direction. Thus, there is a problem in that optical fibers having degraded transfer loss characteristics are manufactured.
It is therefore an object of the present invention to provide improved silica glass products and an improved method for manufacturing a silica glass tube.
A further object of the invention is to provide an improved overclad tube for an optical fiber preform and an improved method of manufacturing an overclad tube for an optical fiber preform.
A yet further object of the present invention to provide a method for manufacturing a tube-shaped silica glass product having a uniform longitudinal cross section distribution.
A still further object of the invention is to provide a method which avoids a non-uniform cross-section distribution due to deformation during zone-sintering.
Another object is to provide a preform which may be drawn into an optical fiber with improved transfer loss characteristics.
These and other objects are accomplished in the present invention by providing a tube-shaped silica glass product and a method for manufacturing a tube-shaped silica glass product using a molded green body having a hollow rod structure. The process contemplates the steps of: (A) forming a molded green body of a hollow rod structure having a uniform outer diameter while having an inner diameter increasing gradually as it extends a top thereof to a bottom thereof; (B) vertically arranging the molded green body in a vertically movable sintering furnace in such a fashion that the bottom of the molded green body is downward directed; and (C) sintering the molded green body while downwardly moving the sintering furnace under a condition in which the sintering furnace is heated to a desired temperature.